The present invention relates to ion implantation systems, and more specifically to the design of high current implanters for serial production line implantation of ions into large workpieces such as semiconductor wafers.
The doping of semiconductors with electrically active elements is now performed almost exclusively with ion implanters. Several recent trends in semiconductor technologies suggest characteristics that would be desirable in the design of ion implanters. These trends include the following:
For workpieces which are silicon wafers, the standard wafer size has increased over the years until 200 mm is the standard diameter used in new facilities today, and manufacturers are planning for 300 mm and larger.
For workpieces which are flat panel displays, dimensions today exceed 300 mm, and larger sizes are to be expected.
As the density of memory elements in DRAMs increases, the implanted energy requirements at high doses are decreasing. High doses of boron at energies between 2 and 10 keV will be required of future process tools.
Processes require the ability to control the angle of incidence of the ion beam on the semiconductor substrates. Variation across the wafer can cause process failures.
High current ion implanters may be broadly defined as the class of instruments in which the ion beam current is great enough that it must be space-charge neutralized to be transported through the ion implanter. This property generally holds true for currents in excess of 1 mA. To meet the usual range of practical applications, high current implanters are typically specified to deliver up to between 15 and 30 mA of n-type dopant ions, and up to between 6 and 12 mA of boron ions. All high current implanters commercially available in 1991 implant batches of wafers, and achieve uniformity of doping by scanning, placing the wafers on a spinning disk, wheel, or drum, to provide one scanning direction, and either moving the wafer carrier normal to the beam, or electromagnetically scanning the beam to achieve another scanning direction.
Batch processing systems of this type suffer from reduced throughput when the batch size is mismatched to the size of the production lot, making it very expensive to test a process, since a complete batch must be processed every time. Rotating disk assemblies must be cone-shaped if centrifugal force is used to retain the wafers, causing variations in implant angle and such variations in the orientation of the implant angle cannot be avoided in spinning disk batch systems. The size of a spinning disk assembly suitable for 300 mm or larger substrates would be prohibitive, as would be the intrinsic value of a single lot. Systems which process each workpiece individually are known as serial systems, and these are to be preferred when the technology permits.
However, because of the difficulty in making a high current beam of high uniformity, commercial machines have relied largely, if not exclusively, on batch processing machines having large motive assemblies to achieve uniformity by spatial averaging.
Accordingly, it would be desirable to develop an ion beam apparatus of high current that is useful for commercial implanting tasks, yet covers a large area uniformly in a serial manner.